1. Field of the Invention
The present invention relates to a method and apparatus for the automatic communication of two telephone subscribers, whose respective telephones are connected to different exchanges, after suitable manoeuvres for placing the two subscribers in communication have been attempted from one of their telephone equipments, these manoeuvres having failed due to the called subscriber being engaged.
2. Description of the Prior Art
The inconvenience caused by the impossibility of a calling subscriber to enter into communication with the called subscriber is known.
As far as the caller is concerned, unless he simply foregoes his call, he can expect to waste an indeterminate, but generally long time in continually repeating the calling attempt until the line is free or until he is discouraged. As far as the operation of the telephone equipment is concerned, there is an overload in the traffic due to the repeated calling attempts. In fact, to the normal efficient traffic, which is sometimes very close to saturation of the equipment, there is added the traffic of repeated manoeuvres of the consumers determined to contact the called subscriber.
To avoid the calling subscriber having to renew his calling manoeuvres until he has obtained communication with the called subscriber, it has already been proposed to effect the automatic call-back of the latter by the telephone equipment itself.
Accordingly, in the device described in U.S. Pat. No. 3,854,014, the caller's exchange comprises a store in which are stored the telephonic identities of the caller and called subscriber and, from these registered identities, it makes periodic attempts to automatic call-back of the called subscriber. Such a device, although it discharges the calling subscriber from the repeated call-back manoeuvres, brings about an overload in the telephone traffic by the periodic, thus random repeated calls which it sends from the caller's exchange to the called subscriber's exchange.
French Pat. No. 2,195,132 relates to equipment in which the calling subscriber, after having dialed the number of the called subscriber, may dial supplementary characters, characterising a desired service, for example an automatic call-back, the supplementary characters being received by a unit located in the telephone exchange of the called subscriber, then stored, and the unit automatically carries out the stored service. When the called number is engaged or the called subscriber does not reply, the caller is informed of the failure to establish communication, he dials the coded figures of the "call-back" and records a message. The line of the called subscriber is periodically watched and the recorded message transmitted thereto as soon as possible. Such a device does not therefore really place the caller and the called subscriber in direct communication, by call-back. It simply transmits to the called subscriber, as soon as possible, a message recorded by the caller. Moreover, due to its technical requirements, such a device can only be applied in private installations.
In Swiss publication "International Zurich Seminar on Integrated Systems for Speech, Video and Data Communications," 15-17 Mar. 1972, Zurich, Switzerland, a system is described in which the caller's exchange stores the identities of the caller and the called subscriber, cylically tests the called subscriber's line and, when this test discloses that the called subscriber's line has become free, automatically calls back the caller, and then the called subscriber, when the caller has replied. The drawback of such a system, according to which the exchange to which the caller is connected maintains the initiative of attempting to establish the communication, is to necessitate a permanent link (semaphore channels) between the called subscriber's exchange and the caller's exchange, the first thus being a satellite of the second. Furthermore, this system is applicable only within a local zone.
It is an object of the present invention to provide a method and apparatus for automatically calling back the caller and called subscriber, without the above-mentioned drawbacks. It avoids any random procedure of establishing the communication, eliminates any abusive traffic or any permanent link between the caller's exchange and the called subscriber's exchange and establishes the communication as soon as the called subscribers are free at the same time.